eudev: only use pragma diagnostics if GCC supports it.

eudev uses GCC pragma diagnostics [1] for some of its logging functions,
to circumvent -Wformat-nonliteral. This feature is only available in GCC
>= 4.6.

The external toolchains for some architectures (PowerPC, SuperH) are based on
GCC 4.5. So eudev will not compile when using them.

systemd also uses the pragma diagnostics, but its dependency on Linux
headers >= 3.8 for the toolchain indirectly forces recent versions of
GCC.

This workaround enables the pragma diagnostics only when using GCC >= 4.6.
This means that if the user uses GCC 4.5 and explicitly sets the options
-Werror -Wformat-nonliteral, the build will fail...

[1] http://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html#Diagnostic-Pragmas



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>